text:
Has a pilot ever accidentally skipped an item in the before takeoff checklists and survived?
I would like to know if there are any documented events where a pilot has accidentally skipped an item from a before takeoff checklist, but managed to avoid an incident, reaching their destination safely.

What kind of documentation will you accept, and must it be accidental or could it also be on purpose?

Happens all the time.  There are only a few items where skipping an item will kill you.  Leaving control locks on, taking off with fuel selected off and the the engine runs until the carb bowl is empty.  Most of the items, it's just an ooopsie. On jets. add some more critical items, like leaving the anti-ice off taking off in icing conditions, wrong takeoff flap setting etc. Even on jets, most of the checklist items are just oopsies if you forget them.  Jets have Takeoff Configuration Warning Systems for the safety critical items if you miss that checklist item.

I was doing a checkride with a new CFI.  We were going to use my plane.  The battery was flat so I went to hunt down a new one from my CAMO.  In the meantime the CFI checked my insurance and realised he wouldn't be covered as P1.  So we decided to take his plane (same model).  By now time was pressing, Rushed through my walkaround and prestart checks, taxi-ed to the hold and did the power checks, took off and stooged around. On downwind, during BUMPFITCHH I realised I'd flown the whole hour without fastening my harness.  I guess this isn't the sort of thing you're looking for?

@JohnK: you can add forgetting to set the cabin pressurization switch to "auto": https://en.wikipedia.org/wiki/Helios_Airways_Flight_522

@757toga yes agreed.  I dunno about others, but on the CRJ there was a big weak link; you select WAI on not until while pulling out onto the runway (LEs get steam iron hot), and it's an action you do after stowing the TO checklist during the lineup. Easy to forget. I'm surprised there weren't more crashes from forgetting.  Later they add a subsystem that cycles low heat to the LEs while taxiing automatically as a safety measure.

@Jim Doesn't need to be anything too formal. Even something like an interview with a pilot saying that they'd forgotten a checklist item would do.

Consider me interviewed then, because I have probably done it a few times and here I am...  You really only hear about these sort of mistakes when something bad happens.  (not sure the "agenda" of your question, but it's a bit like trying to prove a negative)

Forgetting flaps is fun. Two big clues is 1) lifts off later than expected after rotation. 2) begins to climb, just,  and then "mushes" as it is getting out of ground effect.  These are not mentioned in the tech manual but I'm aware of them.  Living or dying can be a matter of not having to maneuver immediately,  lots of runway in front of you, thus having enough time to trigger that subconscious brain-to-hand-to-flap-lever neural pathway.

I'm in this post and I don't like it :)

Skipping checklist items is, unfortunately, something that happens thousands of times a day. Since there aren’t thousands of fatal accidents per day, what more proof are you looking for?

Prior to around WWII, checklists didn't even exist.  They came about because in pursuit of performance, Boeing had created (in the B-17) a more complex aircraft than predecessors, and the prototype crashed because something was forgotten.

... it happens all of the time  =>  No, as @757toga says. BUT there are so many flights worldwide every day that something happens all the time. Each unique chain of events typically determines the consequences of every "something."

Yes, I forgot to preflight the pitot tube, took off, had no accurate airspeed indication, the airspeed indicator acted as a cross between an altimeter and a rate of climb indicator.  The pitot side of the system seemed to be basically sealed up tight, but with a very slow leak to the outside atmosphere.  Ascending to approx 8000', the ASI initially showed a very high value, but very slowly leaked down toward zero, and then when I descended back to the ground two hours later, the ASI showed a negative value.  (Needle was left, or counter-clockwise, of zero.)  I refrained from taking any unusual actions to deal with the apparent negative airspeed.
After landing I noticed the pitot tube was plugged up tight with dried dirt, undoubtedly due to the action of a mud-dauber wasp.  Documentation exists in the form of digital photos saved to my hard drive-- both from mid-flight and after landing.
The event happened more than 24 hours ago, so I hope that counts as "in history".

If you're properly trained and skilled as I'm sure you are, loss of airspeed should be a total non-event, as I'm sure it was.

:-) Loss of airspeed indication might be a non-event, but loss of airspeed is another matter.

“Airspeed not alive… not 60 knots… rotate...”

As a precaution, I would have jettisoned the canopy to facilitate bailing out, except that the aircraft had an open cockpit. : ;  (Joke, but the last part is true, and I was wearing a parachute! )

Oops I guess this deals with preflight while the actual question was about pre-takeoff checklist- oh well--


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Honeywell 880 weather radar malfunctioning
Is anyone aware of issues with the Honeywell 880 weather radar not painting moisture?   We had our original 800 unit go bad on our Falcon 7X,  We have installed 3 replacements that have hardware and software upgrades.  We have inspected the flate plate, radome, wires, bonding points etc.

Shouldn't this question be addressed to Honeywell???

Painting moisture is pretty much all it does.  You mean it will paint terrain if you point it down, but not rain?

Well, it's been exceptionally dry this summer, the weather radar is probably just fine!
Welcome to aviation.stackexchange, Mark! Whilst you might find people here who are able to suggest troubleshooting best practices, I think the best approach in this case is to contact Honeywell.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Single vs Twin motor for an electric RC airplane
I'm designing a fixed wing UAV for a project. I need an airplane that can fly for at least 200 minutes. Obviously, it means a big battery and thus a lot of added weight. I've been debating whether or not I need a twin motor configuration. The idea came after I saw a video of a modified "My twin dream" airplane that has a range of 269 km and can fly for more than 5 hours. The extra thrust might be the reason it can fly for that long but I can not find any research for electric twin motors whatsoever.
Let's say the cruise speed would be 15 m/s. If it only has one motor, maybe it would need to run at 90% of its power vs if it has two motors they would need to be at 45% of their power to match the required thrust for that cruise speed. This could mean, two motors are more efficient than one, even though the extra weight of the added motor.
I'm not sure how to solve this problem.

There's a new Drones StackExchange for this! :)

You should keep in mind that adding an engine adds wiring, extra propeller (a larger single prop might to be lighter in RC than an equal setup of two), overall complexity etc.
As I understand it, electric motors have a "sweet spot", or rather an area of (iirc) certain load and rpm. You should desing your configuration so, that the motor(s) would operate in this rpm/load range in cruise. This sweet spot is (again, iirc) much higher than half load. So it is likely you are better of with a single engine than two that are only utilizing about half their rated pwr.
This sweet spot is very motor specific, so you need to dig deep into spec sheets.

"This sweet spot is (again, iirc) much higher than half load" What is "higher?" RPM, torque, or power? From the RPM-efficiency curves I have seen, the peak efficiency tends to occur somewhere at around 1/7th the no-load RPM. Whereas power peaks at 50% the no-load RPM.

The single prop should be more efficient if both systems are designed properly. My understanding is that the benefit of a twin on UAVs is ease of mounting the payload (counter-rotating props is nice too).
Note that one larger propeller is more aerodynamically efficient than two smaller ones totaling the same area. That is a big drawback in efficiency for using two electric motors instead of one.
From the RPM-efficiency curves I have seen for DC motors, and from what I have read, the efficiency tends to peak at right around 1/7th the no-load RPM while the power peaks at 50% no-load RPM. Better to load it a bit too much and have it run a bit slower than load it a bit too little and have it run faster since the slope on the side that is underloaded drops off much faster than the slope on the side that is overloaded.
If you want to get peak efficiency when it occurs at such a high RPM combined with the efficiency of large propeller, you will probably need a gearbox. Gearboxes are expensive so you probably don't want two of those.
If the mission profile allows, you might benefit from designing the motor-prop for maximum efficiency during a climb and then operate it to repeatedly climb and descend in an unpowered glide rather than design it for maximum efficiency in cruise and then cruise under power the entire time.
A motor-prop combo designed to be most efficient during cruise will produce just enough thrust to overcome the drag so that it can be running the entire time with no efficiency penalty, but such a combination will probably have a lot of trouble getting off the ground.
Designing things for optimal efficiency during climb and then operating like a sailplane lets you use a motor-prop combo that is better suited for takeoff while also enabling you to only ever run it at the point of optimal efficiency during travel.

Your intuition is right, that aircraft design, large or small, is a mess of tradeoffs.  One way to experiment with such designs is software such as the venerable Motocalc, which lets you choose the airplane's attributes (minimum flight duration), some constraints on its design (up to two motors, weight range for batteries, etc.), and then gives you many designs that optimize your attributes.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Pressure coefficient over a flat plate against its length/width ratio
We conducted an experiment to measure the surface pressure distribution around a flat plate (dimensions below). The plate has 104 equally distributed tappings as shown below.

Where the arrows indicate order of measurement.
I would like to plot the pressure coefficient against length(L) to width(W) ratio L/W
The following data is the pressure recorded clockwise (as suggested by the diagram) at the corresponding tap. Saved in a file called "combined_pressures.txt".
The first four rows are pressure data over the upper part of the plate, while the other four rows contain pressure data over the lower part of the plate.
First row from tap 0 to upper tap 12, second row from upper tap 13 to upper tap 25, third row from upper tap 26  to upper tap 38, fourth row from upper tap 39 to upper tap 51, fifth row from tap 52  to lower tap 40, sixth row from lower tap 39 to lower tap 27, seventh row from lower tap 26 to lower tap 14,  eighth row from lower tap 13 to lower tap 1.
100.016 -15.341 -24.538 -29.053 -31.768 -32.851 -33.200 -33.873 -33.885 -33.219 -32.734 -33.104 -32.086

98.884  -16.562 -25.100 -29.238 -31.768 -32.662 -32.833 -33.473 -33.504 -32.833 -32.317 -32.712 -31.540

-30.698 -29.985 -27.909 -26.277 -24.625 -22.845 -20.910 -20.444 -19.227 -17.962 -17.722 -18.021 -17.137

-16.762 -17.085 -16.109 -16.469 -17.106 -17.181 -16.875 -17.638 -17.704 -17.575 -17.722 -19.000 -18.960

-19.587 -20.746 -20.792 -22.206 -24.061 -26.243 -28.981 -35.477 -32.742 -31.288 -31.274 -32.124 -31.540

-31.829 -32.426 -31.656 -31.828 315.657 -32.851 -36.318 -30.867 -27.032 -24.142 -22.726 -22.330 -20.601

-19.587 -19.525 -17.982 -17.950 -17.670 -17.370 -16.508 -16.836 -16.562 -16.223 -16.263 -17.237 -16.772

-16.950 -17.433 -16.671 -17.025 -17.482 -18.125 -18.159 -19.643 -20.559 -22.017 -24.185 -28.402 -30.628

I used the following MATLAB code:
plate_length = 0.225; % m
plate_width = 0.025; % m

pressure_data = load('combined_pressures.txt');

% Split the data into upper and lower parts
num_points = size(pressure_data, 2); % Number of pressure data points

% Define freestream conditions
P_infinity = 101984*(1/101325); % bars
rho = 1.225; % kg/m^3 (air density)
V = 14; % m/s (freestream velocity)

chord_length = plate_length + plate_width;

% Calculate the length-width ratio for each data point
length_width_ratio = chord_length * (1:num_points) / num_points;

% Calculate pressure coefficients for upper and lower parts
Cp_upper = (pressure_data(1, :) - P_infinity) / (0.5 * rho * V^2);
Cp_lower = (pressure_data(5, :) - P_infinity) / (0.5 * rho * V^2);

% Create a plot
figure;
plot(length_width_ratio, Cp_upper, 'r', 'DisplayName', 'Cp top');
hold on;
plot(length_width_ratio, Cp_lower, 'b', 'DisplayName', 'Cp bottom');
xlabel('Length-Width Ratio');
ylabel('Pressure Coefficient (Cp)');
legend('Location', 'Best');
title('Pressure Coefficient vs. Length-Width Ratio');
grid on;

Which yields

This is not what I expected. I expected a plot similar to the following

I am wondering whether I made a mistake with the code or it's due to faulty experimental data.

Three questions: 1) why do you calculate the chord as being "chord_length = plate_length + plate_width"? Shouldn't chord simply be plate_lenght? 2) Why do you use the pressures only from the first and the fifth row of your txt file: "Cp_upper = (pressure_data(1, :) - blabla; Cp_lower = (pressure_data(5, :) - blabla"? TBC

Have you taken into account that the taps are arranged/stored from leading to trailing edge on the upper surface and the other way around on the lower surface? Plus, as already pointed out by @RobMcDonald, being the plate symmetrical and at 0° angle of attack, the pressure distribution on the top and on the bottom should be symmetrical and close to 0

I would recommend you plot the pressure ports as a fraction of length along the plate.  I.e. their 'X' position divided by 255mm.  The plot should start at zero and go to one.
A flat plate like this at zero angle of attack should give Cp=0 everywhere.  This will not be true near the leading edge around the rounded nose section.  However, it should settle back down to zero a bit behind where it flattens out.
The perfect Cp=0 result is also very hard to achieve because it requires the angle of attack to be exactly zero.  Small deviations will throw off the Cp distribution substantially.  Non-uniformity in the tunnel flow can also throw it off.
The chart you're comparing to is mis-labeled.  It shows the negative pressure coefficient, but it does not tell you that is what it is doing.  We often plot the negative pressure coefficient (we flip the Y-axis vertically), but it should be labeled clearly.


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How is rotor airfoil analysis different than fixed wing airfoil analysis?
What the is difference between rotor airfoil analysis and fixed-wing airfoil analysis?
Intuitively, it seems that the airspeed at the tip of a rotor blade would be more than that at its root. Wouldn't this result in irregular torsion along the blade? Subsequently, wouldn't this affect the airfoil analysis as compared to a fixed wing?
If this is the case, then what software (hopefully free) can carry out this analysis?

You are correct on many fronts.  Rotor blades see a varied load along their length, resulting in twisting, bending, and lagging (e.g., the blade being bent against its direction of rotation) loads.  It's a mess.  And, yes, all those effects feed back into blade aerodynamics, but that's a much more complicated answer that you might be looking for.
What you are describing is the foundation of blade element theory.
Let's consider a helicopter rotor, independent of the helicopter fuselage, and look at one annular ring of it while the rotor is in hover (so we have an axisymmetric flow field):

If we look at that annular ring, it would "cut" through a section of the rotor, which, in 2D, would look something like an airfoil.  If we drew a bunch of vectors on it to represent the forces acting on the airfoil and the direction of the relative wind, it does look a heck of a lot like our 2D airfoil free body diagram from fixed-wing aerodynamics.  And, truly, blade element theory (BET) considers it as such.

In BET, elements of the blade (such as the 2D airfoil above) operate without impact on the other blade elements.  If you can determine the inflow at a particular blade section, you can determine the sectional lift and drag, and, integrating those properties across the blade, you can determine the thrust production and torque requirements that correspond to a particular operating condition.  That said, while most of the labels on that diagram are familiar from fixed-wing flight, there are a few that should be different, namely: the induced velocity, inflow angle, and tangential velocity.
As you stated, the horizontal speed of the blade is going to change based on where you are on the blade, radially speaking--which is absolutely correct.  If we look at a distribution of elemental velocities around the rotor, it's going to vary as shown here, in a plot from Leishman's Principles of Helicopter Aerodynamics (which has a great description of how to implement all of this, as well as some basic corrections for going into some rudimentary higher-order approximations).  Note the hover case on the left hand side, where the velocities are axisymmetric, versus the forward flight case on the left hand side, where the velocities are asymmetric.  That second case is a whole 'nother can of worms, and we'll get there in a minute.  Stick with the hover case for now.

So knowing your radial location can get you the element's velocity, and knowing your climb rate and help you get how fast flow is moving vertically through the rotor, but flow has to be moving vertically through the rotor even when the aircraft's not climbing -- else our analogy of a rotor being a big fan (which it is) doesn't correspond with reality.  Hence, there's some inflow speed, and it makes sense that that inflow speed is correlated to the amount of thrust being produced by a given element of the rotor...and this is the crux of the problem for us, since it has to capacity to dramatically change the angle of attack of the blade section (and thus our lift and drag predictions).  So: how do we estimate the inflow velocity?
A simple approximation uses momentum theory (i.e., a quasi-1D analysis on fluid moving through the rotor disk, treating the rotor as a device that simply adds momentum to the flow) to determine the inflow on the rotor along that annular section, assuming that the inflow is axisymmetric.  For what it's worth, it's a simple way to capture inflow velocity variation (and thus angle of attack variation as well) across the rotor disk, which is something that momentum theory cannot do alone, and it's decent at what it does.  It has been the de-facto standard of the wind energy industry for awhile as more complex CFD computations have caught up to the industry's analysis needs and desires.
However, when flow is no longer axisymmetric, we get into a big mess of approximations to determine what that inflow does look like, since the wake will start to interact with the inflow as well and that, somehow, needs to be accounted for.  It is here that we start to depart from analytical expressions and look to computational models using free-wake theory or higher-order models.  Where fixed-wing models advance in similarity by modeling the wing's wake, viscous effects, etc, rotary wing models model a rotating wake that is being convected and recirculated by both rotor effects and the freestream.
That said, the expressions for a rotor in hover are relatively easy to code up, depending on how much fidelity you want (e.g., how much you know about the airfoils used on the blade as well as how the blade tapers and twists along its length).  Googling around, I found a MATLAB code that might be useful, again, depending on your problem: https://www.mathworks.com/matlabcentral/fileexchange/21994-analysis-of-a-rotor-blade-system-using-blade-element-momentum-theory

Can you elaborate on how to estimate the inflow...? I'm trying to derive the rotor thrust (in hover conditions) by integration of the elementary lift, but I have no idea on how to calculate the inflow... In momentum theory, I understand the derivation of thrust as a function of power and rotor diameter, but I see no way to get the inflow via momentum theory...

Other responses are complete but i want to add this:
To avoid torsion dues to speed difference at root and blade tip ,blades are twiste(The twist of the propeller blade is the angle between the chord of the blade tip profile and the chord of the blade foot profile (drawing below). This twist allows the propeller to operate with a relatively constant angle of incidence over its entire length in cruising flight.

The setting angle θ is defined between the profile reference rope and
  the plane of rotation.)

the advancing blade and faster than the reversing blade :so  to compensate for the asymmetry of the lift:

Each rotor blade is fixed to the rotor hub by means of two
  articulations, one in the plane of rotation and the other
  perpendicular to it. The position of the joints is chosen by each
  manufacturer, mainly with regard to stability and control.

The horizontal joint, also called the hinge, allows the blade to move up and down relative to the plane of rotation individually and also be advanced or delayed relative to its direction of rotation (forward / backward movement).
The vertical joint, called the drag hinge, allows the blade to move back and forth. This movement is called offset or drag. Dampers are usually used to prevent excessive back and forth movements around the drag hinge. The purpose of the drag hinge and dampers is to absorb acceleration and deceleration of the rotor blades.
Source


text:
How can this distance on the wing be calculated?
To calculate a wing's Mean Aerodynamic Chord MAC, I need the length of the line in blue in this image:

How can I calculate this distance?

I’m voting to close this question because it's a math (trig) question & not about aviation, the background image notwithstanding.

Ignore the part of the diagram above the blue line.
Assuming that the trailing edge at the wing root is at right angles
to the fuselage,
then 6.5 and 23.5 and trigonometry tell you everything about the triangle
that is like a delta wing whose root lies on the midline of the fuselage.
Now you know the length of the delta wing's trailing edge.
So use similar triangles.
Divide 5.6 by the length of the TE.
Multiply that by 6.5 to get the blue line's length.
(Or just use a ruler.  The diagram's numbers are only to two significant figures!)

mmm I don´t  understand all

Similar triangles? I don´t see the geometry that two triangles

Make the delta wing.  Then you have the first triangle within which you make the similar triangle.

If you can assume the sweep angle is constant from root, then
6.5  - 5.6 * sin(23.5) = 4.26

if you don't explain what's happening, this answer is essentially useless, as it does not actually help to understand.


text:
When must a pilot report Negative Contact?
Which regulation or AIM section specifically states when a pilot is required to report negative contact?  It appears that different airports in the US have slightly different expectations.
My understanding is that ATC is only relieved of separation requirements when the pilot says "Traffic in Sight". However, if ATC says "Cessna 123, Number 2 following traffic on downwind", if pilot says "Number 2 following traffic on Downwind, Cessna 123", that is the same as saying Negative Contact. In other words, unless the pilot says the magic words "Traffic in Sight", ATC treats anything else as negative contact. Until the pilot reports traffic in sight, or ATC says "Report Traffic in Sight", it seems like unecessary radio traffic for pilot to report negative contact, unless specifically asked by ATC.
Also, what do tower controllers think about this?

The short answer is that in your example there's no reason to assume that the controller was asking the pilot to maintain visual separation. If the controller had wanted the pilot to do that, they would have asked if the pilot had traffic in sight and the pilot would be expected to answer the question.
The ATC orders have an entire section on visual separation (section 7-2-1) with lots of details. There are two types: ATC-applied and pilot-applied, depending on who's actually responsible for maintaining the separation. I think you're asking about pilot-applied separation, and there's really nothing in the dialog you quoted that suggests the controller intended the pilot to maintain their own separation. In fact, it's possible that the controller was maintaining visual separation themselves based on seeing both aircraft, and that doesn't require the pilot to have the other aircraft in sight.
If ATC assigns pilot-applied separation, they need to hear two things from the pilot: they have the traffic in sight, and they will maintain visual separation. See 7-2-1(a)(2)(c):

If the pilot reports the traffic in sight and will maintain visual
separation from it (the pilot must state both), the controller may
“approve” the operation

The Orders have an example and it's laid out as a three-step process: first the controller tells the pilot where the other aircraft is, then they ask the pilot to confirm traffic in sight, and finally if the answer is positive they ask the pilot to maintain visual separation. Since ATC is asking a question, the pilot has to answer one way or another, whether that's with "traffic in sight" or "negative contact".
Coming back to the example you gave, the controller simply told the pilot that they were number 2 behind other traffic. The controller didn't ask if the pilot had traffic in sight and while the pilot might volunteer that information, the controller didn't need it at that point in time (they might need it later if the aircraft are getting too close, or something else changes).
Long story short, if ATC needs to know if you have the traffic in sight then they'll ask you.

Great answer!  This was my understanding that if they wanted me to report traffic in sight they would ask, which I usually volunteer anyway. But when its not in sight I usually don't report it because its assumed, unless they specifically ask. I'm flying at a busy field where it appears that ATC expects pilots to report both Traffice in Sight and Negative Contact without being asked. Maybe I'll call the tower and ask them if its a local thing.

@Devil07 Thanks, I’m glad it helped! There are lots of informal local practices out there and calling the tower wouldn’t hurt. If your field is busy and especially if it’s a lot of training traffic then ATC may be more concerned about who can see who.

Pondlife has a good answer for if the both aircraft are IFR. In that case visual separation is used in lieu of other standard separation (radar, non-radar, or vertical separation), and in that case the "traffic in sight"—"maintain visual separation"—"wilco" sequence must happen before standard separation is lost.
But for VFR aircraft, at least outside of Bravo airspace, we don't actually have ANY minimum separation. Obviously it would be very bad if two planes hit. But until they hit we don't have a "deal" (loss of separation). Look in the 7110.65 chapter 7 under Class C VFR service and Class D VFR service. In Class C, we have to have "target resolution" (dots on the scope don't touch) between a VFR and an IFR aircraft, but there is no standard separation VFR-VFR. In Class D there isn't even target resolution. All we have to provide are traffic advisories and safety alerts.
What does that mean for a VFR pilot? If we call traffic in the downwind and say you're going to follow them, we assume you have the traffic in sight and will proceed under standard VFR/VMC "see and avoid" procedures as required in Part 91. The back-and-forth "will maintain visual separation" isn't required. And even if one of pilots is actually IFR, if the tower can see both planes and issues proper control instructions, for example "follow the traffic on final," tower-applied visual separation can be used without going through the whole sequence.

edit: to actually answer the original question. If you get traffic called, you should always respond with "looking" or "negative contact" or "traffic in sight." ("Got him on the fish finder" is completely useless and unnecessary.) Because of what I said above, we do different things based on if you have the traffic in sight or not.
There are also some mandatory traffic calls we give even if you don't need to do anything about it, for example merging target procedures say that for certain types of aircraft/flight plans we have to call traffic if the targets will merge and we don't have more than the required vertical separation. This is for your situational awareness so you don't freak out if the TCAS starts yelling at you, but we don't actually need you to do anything because the minimum separation is still legal separation. Even in that case a quick "looking" is good so we know you heard.

Good answer, and welcome to Av.SE!  Always good to have more controllers on the board to get that perspective on things.

@randomhead thanks for the contribution. I'd like to clarify your second to last paragraph. Isn't "looking" and "negative contact" treated exactly the same by ATC? Also, if the pilot doesn't say "traffic in sight" doesn't ATC keep providing advisories until the pilot sees the traffic or they ask "report traffict in sight" or "do you have traffic in sight?"  I'm really interested in your point of view.

@Devil07 yes, I think "negative contact" is technically correct but you can say "looking" if you like, it's shorter. Same difference on our end. And yes, we will continue to provide advisories (or otherwise control the situation, e.g. extend downwind) until you report in sight—but we want you to say something to acknowledge the transmission, hence "looking."


text:
Is a chambered fan more efficient than a ducted fan?
A chambered fan is not exposed to the oncoming airflow in forward flight.
I am working on a VTOL design that uses a chambered fan.
There will be a wing attached to top of the thrust arms.
Could it produce 12,000 pounds of lift force using a GE T901 with 3,000shp with 8' dia. contra rotating fan?

How does air flow through this apparatus?  Which way through the disc-shaped grill?  Through the four pipes, if that's what they are?  If so, why four and not twenty?

The grill around perimeter of craft is the intake. Contra rotating push prop fans are located in the center fan chamber.  The fan forces air into the four thrust arms, splitting the air flow and spreading the air flow out for 4 points of balance.

There are only four thrust arms instead of twenty thrust arms, because it only takes four to have great balance like a quadcopter. The steering is controlled by swivel nozzles at the 4 thrust arms exit ports.

Differential thrust, e.g. from vanes at each arm's intake, would be more efficient than vectoring the thrust at each arm's exit.

The design is not very efficient:

It does not use forward airspeed to pre-compress the airflow entering the engine.
It blows exhaust air into the air inlet.
It bends the exhaust flow 180 deg, creating friction and loss of thrust.

Short, straight, unimpeded airflow works best.

Here is a link that shows a straight pipe compared to U shaped bent.https://www.facebook.com/photo.php?fbid=3651990474826861&set=pcb.3651998764826032&type=3&theater

Do those screenshots claim some advantage for a U-bent pipe?  If so, what?

Yes the straight pipe put out 2,457lbs of force and the U shapped pipe put out 4,736lbs of force. Maybe someone else could do a calculation?

If adding a U to a pipe doubles its thrust, why don't we see them on many aircraft?  The bolder the claim, the stronger the evidence needed to support it.  Step away from the computer and measure something physical.

It's a Facebook page. It shows two long pipes, which is never the best due to boundary layer effects. It only states Pressure, while maximum thrust is created at maximum exhaust stream velocity - when the exhaust stream expands to ambient pressure (maximum exhaust gas velocity). A collection of half truths and funky special effects that draw attention. A piece of engineering in the manner of Twitter.

I think the U shaped pipe needs to be slightly funneled and not a perfect dia. straight pipe. 45* and 90* turns causes air to tumble and swirl. When the funnel shaped pipe is bent in a U shape the air flow sweeps with minimum turbulence. If the pipe where more flat instead of round, the shape would be more like a wing. There will always be skin loses, but you gain steering of the craft, with four thrust arms. The  4 thrust arms do not have to be at 90* like most quadcopters, they can be closer together maybe  45* spread on each side of craft.

In aviation there is no generally accepted meaning for the term "chambered fan," so there is no way to claim in general that a chambered fan is or is not more efficient than the corresponding ducted fan.
In fluid dynamics, straight pipes are more efficient than bent ones, so the 180 degree bends in the four exhaust pipes (and between the intake grill and the fan) are less efficient than the corresponding ducted fan design, whose intake is at the top and exhaust is at the bottom.
If this chambered fan is shielded from the airflow induced by horizontal flight, then it cannot exploit translational lift, so it would be less efficient below about 50 knots.  But for horizontal flight the aircraft must tilt in that direction, pushing that airflow into the circular grill intake.
Some of the thrust leaving the four pipes would be entrained into the grill intake just below, causing at best some inefficiency, and at worst a partial vortex ring state.
The simplest way to compare the thrust and efficiency of this design to the corresponding ducted fan design blowing directly downwards would be to build a small model and measure it.  Radio-control model suppliers sell cheap "EDF" electric ducted fans, and the ductwork could be 3D printed.

Here is a pic of craft with a wing and intake at rear of craft.https://www.facebook.com/AviationLakeHopper/


text:
What is the airplane with the best flying characteristics?
I'm reading Stick and Rudder by Wolfgang Langewiesche, and in Chapter 7 titled, "What the Airplane Wants to Do" he discusses stability. In discussing it, he says that no plane yet built is completely "well-behaved" and that the plane closest to perfection is Fred E. Weick's Ercoupe.
It's been over 50 years since this book was written. Have aircraft flying characteristics improved? How much have they improved? And what airplanes have we built that have the best flying characteristics (which are the "best-behaved")?

As stability is always at odds with manoeuvrability, it’s an important design objective to not make the plane too stable...

Yeah I can cut a rock to the shape of a cube and set it down on the prairie, and it ain't goin' nowhere but it's stable as all get out.

Is this question talking about flight mechanics isolated stability or are you including in your question also the electronics? Currently, if we consider the full stability of the full airplane, including the electronics, the stability has increase significantly.

Essentially, I'm asking about how "well-behaved" (to borrow Langewiesche's term) modern planes are as compared to the ones available when the book was written, however the favourable flying characteristics were achieved. I think I'll rephrase the question.

This is a pretty broad and subjective

@Carlo, very broad but not entirely subjective. There are control laws that are 'best', i.e. are proven to give best results. The problem is, this highly depends on the task at hand. This is one of the reasons why computer-controlled (and even some older) aircraft have reconfigurable control laws. I don't have the guts to write an elaborate answer, but to the first approximation, we humans control best when there is one integral between the input and output. Typical roll control is the closest to it: the stick input is roll rate, the aim is roll angle.

A Piper Comanche once landed itself in a field to save a pilots life i'd say thats a well behaving airplane

@Zeus best for what? Best for a primary trainer that needs stability to assist trainee pilots in learning the ropes? Best for an aerobatic competition flyer who needs wild sharp cornering? Or best for a fighter pilot, airline pilot, bush pilot? They all have different, sometimes wildly different, requirements and thus wildly different definitions of "best".

@jwenting, that's what I said: depends on the task at hand. But when you break down the task to the basics such as 'tracking a dynamic target' or 'stabilising this or that flight parameter' etc, there are broadly known 'best' laws. This can be objectively measured. As Steve pointed out it his answer, Airbus comes close to that for many piloting tasks. But there is an additional human factor here: experienced pilots trained on 'difficult' systems often dislike 'simple' ones; when we can predict the system's behaviour, we enjoy making predictive corrections.

First, let's get a little perspective
Stick and Rudder was written in the early days of aviation. To a modern reader, things were really weird back in the early days of aviation.

For a while, air traffic control was a guy sitting in lawn chair next to the runway waving a big flag that said "GO".
Parts on the plane just failed.  Planes would crash on their way back from bombing Hitler, a truck full of mechanics would come out to bolt on some (hopefully) new parts, and then they'd take back off in the crashed airplane to go bomb Hitler some more.
One time, the Allies bombed Switzerland by mistake. This was a world where more than one airplane could miss not only a target, but the entire country.

The kinds of things that went on in those days as a matter of routine would send any modern-day NTSB investigator into full-blown panic attacks.
What I'm getting at here is that it's a mistake to judge Stick and Rudder by modern standards.  This is not the equivalent of brain surgery.  You have to think of this as a surgeon just trying to get other surgeons to wash their hands with soap before they operate.
Now on to your questions

Have aircraft flying characteristics improved?

Yes. One example out of many:  Cuffed wings.

How much have they improved?

Enough that Wolfgang didn't need to write a second book.

And what airplanes have we built that have the best flying characteristics (which are the "best-behaved")?

An Airbus flying in Normal Law.
There's a section in Stick and Rudder (page 160 in some editions) where Wolfgang says that a "foolproof" airplane is one in which the controls have been restricted so that the pilot is mechanically incapable of pulling the airplane to an angle of attack which will result in a stall.  Guess what?  A pilot flying in normal law can yank on elevator all day long and not stall that Airbus.
An Airbus in normal law automatically trims itself.
An Airbus in normal law automatically increases power on the engines if a pilot pulls the airplane into a high angle of attack.
Some people might hesitate to put words in a dead man's mouth, but he's not here, so I'll go for it:  The best behaved airplane according to Wolfgang would be a modern-day Airbus airliner.

Thanks! I know this wasn't the most specific or well-informed question, but you still gave me the answers I wanted (and made me laugh).

Flying characterisctics have improved as engineers have learned more about aerodynamics. Like adding wing dihedral to make the plane self centering.  Better stall characteristics.
How much?  Tons! Enough so so that manufacturers could dream of a plane in every garage for homeowners.
Which best?  Cessna Cardinal after getting past the initial year jitters. Sweet handling, pretty fast, good on fuel, easy to land.


text:
Take off with 0 kt ground speed
Can an aircraft take off with zero ground speed?

Theoretically, a lot of things are possible, but for that, the main criterion would be to have a relative wind equivalent to the minimum liftoff speed of the fighter. Which seems to be about 200km/h (at least for Rafale, F15, A10 according to wikipedia), so it is quite unfeasible in practice

Just to clarify: You are not talking about VTOL (vertical takeoff and landing) here, are you?

Relevant video: Piper Cubs at Air Force Academy in strong winds

Does this answer your question? Can planes take off from a treadmill?

The speed in relation to the ground is completely irrelevant. The only thing that matters is the speed relative to he air. (It's called an "airplane", after all.)
So, if you can get enough air flowing over the wings without moving the airplane relative to the ground, then it will take off. The takeoff speed of an F-22 Raptor, for example, is somewhere around 130 kt ≅ 150 mi∕h ≅ 240 km∕h, so the required wind speed would be equivalent to a category EF3 tornado.
If you can generate that much airflow, then yes, the jet will take off.

If there is enough headwind, yes, as that's the only thing that matters, groundspeed is of no relevance.
If you're asking about the treadmill, see here.


text:
How do airplanes with thrust lines above CG still fly without a down pitch moment?
Mainly the Icon A5. I remember trying to make an RC aircraft similar to it, but the moment i'd spin up the motors the aircraft would understandably pitch down. i couldnt fly it at all.
So how do these planes with thrust lines way above the CG fly? I've seen some people say its an interaction of the airflow above the horizontal stabilizers, but it hasn't yet clicked with me.

There are a number of airplanes where the horizontal stabilizer provides a significant downforce in flight, either by being airfoil shaped, or angled downward, or both.  The tuck-under tendency is greatest at high thrust and low speed, so the elevator will be designed to ensure sufficient pitch authority for that, but the plane should be designed so that in cruise, the aerodynamic downforce of the tail equals the nose-over torque of the high engine mounting.  I suspect high-engine aircraft often have larger than usual trim tabs as well to ease pilot workload.

What you generally do with a high thrust line is place the horizontal stabilizer surface directly in the propeller wash to help offset some of the trim change caused by thrust changes.
When you add power, the increased velocity over the tail induces a nose-up trim change by increasing trim down-force, which compensates for some of the nose down trim change induced by the increasing thrust on an axis that is above the C of G.
The Lake Buccaneer uses a substantial semi-rigid (that is, adjustable fixed) trimming surface at the outboard ends of the stabilizer, to create an instantaneous increase in trimming downforce to offset the nose down thrust line effect.  It helps a lot with the Buccaneer, but doesn't eliminate the thrust effects entirely, and adding power still causes a pitch down, and vice versa. Just much milder than without the Buccaneer's stabilizer configuration.
The A5 appears to have the thrust line much closer to the vertical C of G than the Buccaneer, and the horizontal tail appears to be sufficiently within the propeller wash to provide some of the pitch offset effect, to the extent that thrust effects on trim are a problem with the A5 (probably much milder than the Buccaneer).
So if you stick the horizontal tail directly behind the propeller in your RC model, you will reduce the nose tuck with power, but you probably won't be able to eliminate it and you're going to have to add an additional channel for an adjustable pitch trimming surface if it doesn't have one.

How do airplanes with thrust lines above CG still fly without a down pitch moment?

For straight horizontal flight, the total pitching moment needs to be zero. With a thrust line above CoG, the thrust vector will always cause a nose-down moment which will need to be compensated by an aerodynamic nose-up moment - trim the nose up with a tail-down aerodynamic force.
Which is fine for constant thrust, it’s just a nuisance that the plane needs to be trimmed every time that the thrust is set to a different level. Indeed placing the horizontal tail in the prop wash will provide the necessary change in trim to an extent.

Here a solution for a related topic, what if thrust line is below the CG (Sukhoi Su-27):
By tilting the nozzle down to point the thrust vector to CG:

At some stage of the design evolution, the engine core is tilted 2 degrees on the front and the afterburner is tilted down 3 degrees on the rear. In total, the engine has a 5 degree bend in the middle.
SU-27 Fighter: Beginning of Story: Part 1
by Nikolay Gordyukov Pavel Plunsky, Vladimir Antonov, Vyacheslav Zenkin, Ildar Bedretdinov
(Picture is taken from a Chinese translation of this book, Chapter 5, Figure 5-43. Top to bottom, 1972 1973 and 1976 layouts.)

Do you happen to have a link to the Chinese translation? I would be interested in it

@GrantCurell ISBN 9787516511565 and ISBN 9787516511572

謝謝我總是需要新的書來練習我工程師的詞匯


text:
Does the Zenith have anything to do with the read-out of an IRU accelerometer?
Let's say that I have an accelerometer as part of an inertial reference system. Will the position of the Zenith with-respect-to the current position (in terms of rotation) of my accelerometer, have any impact on the accelerometer readings? Is there any kind of relationship at all?
Or, in other words, do we ever speak of accelerometers as part of an IRU... in terms of relationship with the Zenith?

@BowlOfRed That is exactly what I mean. Let me edit this question, hopefully things will be a little more clear.

I'm not sure what you're asking here, can you clarify what you mean by the "position of the zenith"? Are you using some specific point of reference?

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text:
What is the encoding difference between ARINC 429 BNR and BCD formats?
I think I have an understanding of the differences between the types of ARINC word formats, but would just like to get some input to see if my understanding is correct. In the ARINC 429 specification, it is discussed that there are some differences in how messages are handled between the two formats.
Data Field
This one just seems like basic arithmetic concepts and doesn't concern me as much. From what I understand, the BCD data-field works the following way:
Data-Field = 0x12340, means that the data is actually 12,340 (twelve thousand, three-hundred-forty) in base-10.
Then, BNR data-field would work like:
Data-Field = 0x7FD1C, means that the data is actually -740 (based on two's complement arithmetic).
Sign-Status Matrix Field
The operational codes just mean different things. Or in other words, the code numbers are the same, but the codes themselves have different meanings for BCD messages than BNR ones. Now, if there aren't any other differences, I would also like to know why the op-code schemes are slightly different.
My Question
So, I guess in a single sentence my question would be: are these the only differences between the types of ARINC messages?

Your basic decoding strategy looks correct -I didn't double-check the values, but the logic is sound. Note, however, that the difference of BCD vs two's-complement is not the only difference between BNR and BCD (and Discrete labels, which are also common). Binary-coded decimal (BCD) and binary (BNR) labels generally do decode as you have shown, but the Sign-Status Matrix (SSM) plays a large part in interpreting that data and using it is not optional.
For BCD labels, the SSM informs whether the data is positive or negative (or what direction is points), hence "Plus, North, East, Right, To, Above" vs "Minus, South, West, From, Below". It also informs if the data is invalid ("No Computed Data"). BNR labels include sign information already in the two's-complement encoding, so duplicating that information into the SSM would waste valuable bandwidth. The SSM still informs you of invalid data, but can also inform of (non-)operation in reversionary mode ("Failure Warning") or correct behavior ("Normal Operation").
What that of that means for a given LRU (line-replacable unit)1 can be completely different from any other LRU. To have any confidence in your interpretation of a label, you must obtain a hardware integration guide for the specific LRU in question to determine what format data is or must be provided in and what the SSM values imply. This integration guide is a separate document from any user or maintenance manual you may have received with an LRU for installation into an aircraft. It is usually only provided to original equipment manufacturers of other LRUs.
For example, a BNR label might still contain a readable value when the SSM is set to "Failure Warning", but whether or not you can still use that data depends on what the failure implies for the particular LRU. I once encountered an LRU that provided a label for barometric altitude that reverted to GPS height above ellipsoid when failure mode was set. This change in represented value was not appreciated at the time, but did teach me to always inspect the SSM first before believing any particular value. Furthermore, many BNR labels are also scaled by some arbitrary factor to provide a wider range of representable values. The scale factor is provided in the integration guide, and is usually a fixed value, but I have encountered some labels with values scaled by values from other labels.
The format for Discrete labels is always specific to the LRU. To know what this encoding is, you must have the integration guide.
Ultimately the difference between each particular ARINC429 label will depend on the specific equipment to which you are interfacing. The standard data types (BNR/BCD/Discrete) provide a rough idea of how the data is packaged, but the integration guide is the authoritative source for data encoding.
1: I use LRU as shorthand to mean any ARINC429 node. In practice, many ARINC429-compatible devices are not necessarily line-replacable.

Would there ever be an instance where the SSM is used as space for the data field (when in BCD) similarly to how this is done for the BNR-formatted numbers?

I have never encountered a BCD label which overrode the SSM for use as additional data space. I would strongly suspect that any device which needs more data space in a new revision to change a BCD label to a BNR label rather than extend the BCD. This change would provide a much larger boost to available encoding space. That said, there may still be a device manufacturer that has elected to use a non-standard BCD label in stead. Any such exceptions should be well-documented in the integration guide.

I pretty much had that inclination, and know for a fact that BNR is sometimes overriding the SSM.

As you may notice from this spec tutorial or this errata report,
the two bits that serve as a "sign-status" field for BCD data
show only status, not sign, for BNR data.
The status codes for BNR data include a "failure warning" code;
it is not necessary to signal a failure warning in the
sign-status matrix of BCD data because the individual digits
can be set to 1111 (four consecutive 1s) in order to indicate
that the values are unreliable.
Also, when latitude and longitude are encoded to 0.1 minute
in BCD format, a different BCD format than usual must be used.
See the commentary in section 2.1.2 of
the errata report.
The positions of each digit's encoding are moved 2 bits
so that the first digit has maximum value 1 instead of 7,
and bits 9 and 10 are used for data.

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text:
What is this flying boat I just saw fly over?
I just (1:30 pm BST) saw a flying boat fly over my house (Fareham, south coast UK). Four engines, very low engine pitch, a flying boat hull, something like a Short Empire. Not a Catalina. Unfortunately, it was too brief and distant for me to get a tail number.
It was primarily painted blue, with a darker blue upper part. I think I could see an RAF roundel on the fuselage under the wing, except that wouldn't be the usual place for one.
Does anybody know what this is?

According to FlightRadar24 it is a 1951 Grumman HU-16 Albatross N98TP, the only flying example of its type. Here it is directly over Fareham at 1.30pm local time today, before heading out over the Solent:
 source
 source
There have been various reports of it in local newspapers and forums.
It belongs to TP Aero which has lots of videos and information on the aircraft.

I think that must be it, except I could have sworn it had four engines. But yes, the yellow flash on the tailplane matches. And what I thought was an RAF roundel was the undercarriage.

From the ground, at a distance, the sponsons might have looked like a second engine on each wing.


text:
How do thrust reversers in high-bypass turbofan engines counteract initial thrust?
I've been searching the internet for awhile and haven't really come across a good answer for this question. So:
I have a basic understanding of the principles of flight (thrust, lift, drag, etc). But one thing that has been confusing me is how exactly a thrust reverser on a high-bypass turbofan engine works. So from what I understand, the large fans at the front of the engine will provide the majority of the thrust. Air coming through the inlet nozzle produces an action-reaction pair on the fan blades to produce this forward thrust. So upon deploying the thrust reversers, the air that has already produced a forward thrust on the fan blades now moves back through the bypass duct to strike the thrust reverser, thus redirecting the air forward by creating another action-reaction pair to slow the aircraft.
So it just kind of seems to me that the thrust reverser just cancels out the thrust from the fan blades. Or is the air coming off the fan blades is accelerated so that by the time it hits the thrust reverser the forward directed thrust is greater than the thrust generated from the fan blades, thus producing a net reverse thrust?
Or maybe I'm just looking at this wrong and need to think of the entire engine as a closed system so that the air pushed backward from the fan doesn't produce an action-reaction pair (and thus thrust) until it exits the engine?
I'm just kind of confused on the specifics of the physics of the thrust reverser (where the action-reaction acts).

Welcome to Av.se!  Good question.  Consider that forward thrust is much more about air being pushed out at high speed/pressure/temperature, than it is about air being sucked in. The vector of that exhaust (aft - forward thrust, or forward for reverse thrust) is much more significant than the pseudo thrust vector created by pulling air into the inlet. I'm sure others will expand on that concept in their answers.

I wasn't necessarily referring to the air being sucked in, but more to the fact that thrust is generated by the fan blades pushing air molecules back and the action-reaction that comes with that. So after an air molecule gets pushed back by the fan blade (thus pushing the airplane forward), it travels back in the bypass duct until it hits the deployed thrust reverser (TR). At that point it hits the TR and is deflected forward, thus pushing the airplane backwards. But this backwards push needs to be greater than the forward push at the fan blades in order to have a net "reverse thrust".

Imagine a (working...) shotgun with the barrel sporting a 180° bend. In which direction does the recoil act, and why?

Some of the imparted energy compresses the air rather than accelerating it, this energy is then available to accelerate the air as it is released from the reverser in the forward direction. And "inlet nozzle" is an oxymoron, inlet is inlet, a jet comes out of a nozzle.

There are several ways to describe thrust, which amount to the same thing. Very broadly, it's described by Newton's third law: if A moves forward, B moves backward with equal momentum. On a closer level, it comes from pressure differences across the surface of a body. Even closer, you'd be looking at viscosity and velocity, laminar and turbulent flow, boundary layers, analytical equations.
But all these ways come to the same result: if one, applied correctly, predicts result X, the rest can't predict an opposite result. In the most basic view, the reverser's thrust comes from pushing air forward. As long as the ultimate result is air being accelerated in some direction, however it happens, the thrust will be in the opposite direction.
If you want to get into the mechanics (which isn't quite necessary), the exhaust produces high pressure between the engine and the thrust reverser. This pressure acts on the thrust reverser's inner surface. The resulting backwards push is indeed greater than the fan's forward thrust.

The key here is that the jet exits with forward velocity substantially greater than the velocity of the aircraft. If the thrust reverser just turned the flow 90 degrees out so that the forward component of the jet was approximately equal to that of the plane, then the original post would be somewhat correct about net thrust forces canceling out. Now how does it manage to get this net result is another thing but I think it may actually be what the original poster was asking.

If you want to separate the effects of the fan and the reverser, the reverser is not just decelerating the fan air flow. It is redirecting the air outwards, but also forwards. This means there is a force applied on the reversers to partially reverse the direction of air flow.
You can also look at the system as a whole. When operating normally, the fan accelerates air aft, providing thrust. But with thrust reversers, that air ends up going outwards and forwards, providing a net reverse throust. You can think of it as a form of thrust vectoring. It's the resulting direction and velocity of the air that determines the thrust and its direction.

So let's just say that the instant that a mass of air reacts with the fan blades a forward thrust of 1000 Newtons is produced. This air then travels back through the bypass duct until it runs into the thrust reverser panels. In order to produce a net "reversed" thrust, this air must react with the thrust reverser and produce over 1000 Newtons of rearward thrust. So what I'm asking is, how is it that the air reacting with the thrust reverser can produce a higher magnitude thrust than what it does coming off of the fan blades?

Some or most of the air coming off the fan blades gets redirected from aft to forward and out. That air is no longer providing forward thrust.

Yes, but the forward thrust is happening directly at the fan blade. So as I was talking about above, the instant that the air reacts with the fan blade, forward thrust is produced. Down the line, that air then hits the thrust reverser. That air hitting the thrust reverser doesn't stop the forward thrust from acting at the fan blades.

Correct;  and so the deployment of the thrust reversers could result in the engine being ripped apart.  But the net effect, after the short start-up transient is that air is entering the engine at rest, and leaving in the forward direction.  Momentum added to the air equals momentum lost by the engine/wing/aircraft...

Have you seen someone propel themselves on a skateboard by using a CO₂ fire extinguisher? Imagine what happens if you replace the nozzle on the extinguisher with one that's U-shaped. The CO₂ comes out forwards instead of backwards, so it propels you the opposite way.

I appreciate the responses, but I don't think anybody is really understanding what I'm asking here. Going back to my first comment, if 1000 Newtons of forward thrust is produced directly at the fan blades, there must be something that ensures that greater than 1000 Newtons of thrust is resulting from the air being deflected from the thrust reversers. If 1000 Newtons of thrust is coming off the fan blades just to have that same 1000 Newtons reacting off the thrust reversers, then there is no net effect. So is the air coming off the fan blades accelerated before it reaches the thrust reversers?

You make a fundamental mistake in the original setup that I think is the source of your confusion.
You state that the thrust from the fan is an action/reaction pair on the fan itself. That is fundamentally wrong. It is the action of the air accelerating out of the rear of the engine that is producing most (???) of the thrust due to the fan.
Forget the fan for a second and consider a turbojet. In this case the compressor stages are slowing the air coming in, and if there's a ramped intake, it's slowing it as well. All of this is actively removing momentum from the aircraft. And yet the aircraft moves! Why? Because the acceleration of the air at the back of the engine makes up for all of this.
So forget all the stages and what-ifs. Thrust is the net difference between the input air momentum and output air momentum. The air starts at the front of the engine at velocity X, and exits the reverses heading (somewhat) forward at >X. Don't overthink it!
p.s. I should point out that if you watch films of early jets that had landing parachutes, you'll notice they always drop them while they're near the end of the runway. If they don't, the net thrust is zero and they couldn't taxi.

This means turbojet engines invented air multiplier technology before dyson!!!

You are correct; the air being accelerated rearwards by the fan blades of a turbofan does exert a large forwards reaction force on said fan blades (this is, in fact, how the vast majority of a high-bypass turbofan's thrust is produced).  And, if the thrust reversers merely slowed the air back to a halt, they would produce no net slowing force.
However, since a turbofan's thrust reverser is physically attached to said engine, either directly or indirectly (via the nacelle and/or wing structure), the forwards reaction force exerted on the engine via its fan blades is also exerted on the reverser... which (when deployed) then exerts this same force on the air which hits it.  Then, as the air cannot pass rearwards through the structure of the reverser, the reverser drags the air forwards with it until the air escapes out the front and sides of the reverser.
A deployed thrust reverser would still slow down the airplane even if it were mounted on an empty nacelle with no engine contained therein, since you're basically dragging a bucket through the air open end forwards (this is easier to visualise with older-style "target"-type reversers, but it's still valid even for the newer types; you just have to imagine a toroidal [doughnut-shaped] bucket).  Mounted on an empty nacelle, this bucket takes air at rest and accelerates it forwards, generating a backwards reaction force on the bucket (and, thus, the airplane).  Put an engine in the nacelle (any kind - all that matters for our purposes is that it accelerates the air rearwards), and the air's journey becomes more complicated, as the air is first accelerated rearwards... but, in the end, all this means is that:

The air hits the reverser - and, conversely, the reverser hits the air (thank you Newton) - much harder than it would have sans engine (this is how the forwards reaction force on the engine is still completely cancelled out).
As the mass flow rate through the engine is far greater than it would be through an empty nacelle (which is why a running turbofan has to actively suck air - and, occasionally, other things, such as tumbleweeds, baggage containers, or ground personnel - in from in front of it, which an empty nacelle doesn't), there is, correspondingly, more air per unit time hitting the reverser behind a running engine, and, thus, being redirected forwards by it, than would hit a reverser behind an empty nacelle (this is how, when the reversers are deployed, a higher throttle setting produces more braking force off the reversers than a lower throttle setting does).

There are a series of picture here where you can see how the bypass air is redirected forward to help slow the plane down
https://upload.wikimedia.org/wikipedia/commons/d/df/F-GTAR_Air_France_%283698209485%29.jpg

This doesn't answer my question about the physics of the action-reaction forces.


text:
How do you calculate coefficient of drag at an angle?
I am trying to make a simple 2d drag simulator for an object in freefall with an x and y component of wind. If I knew the coefficient of drag on the bottom and side of the object could I use theta to get the approximate CD at an angle?
Probably and oversimplified function but something like:
$CD_{\theta} = CD_{bottom} + (CD_{bottom} - CD_{side})*\frac{\theta}{\pi/2}$

Don't forget the object will accelerate to constant velocity of any cross wind and then have no angle to the relative wind.  Also, in spite of the rectangular shape, when released, there may be a "lift" component to the left, as it has an Angle of Attack (which grows less and less in the free fall, becoming drag only at the base).

You should transform the title in a question as this is standard on this Q&A website.

Let's try to derive something. Assume the x-axis points to the right and the y-axis points up. I'm gonna use the subscripts b and s for bottom and side. The incoming velocity vector is decomposed in two components along the axes, so $v_s=v\sin\theta$.
Writing the total drag force as a vector:
$\vec D=\begin{bmatrix}-D_s\\D_b\end{bmatrix}=\begin{bmatrix}-C_{D_s}\frac{1}{2}\rho S v_s^2\\C_{D_b}\frac{1}{2}\rho S v_b^2\end{bmatrix}$
Taking the magnitude of the vector:
$D=C_D\frac{1}{2}\rho S v^2 = \sqrt{{C_{D_s}}^2\left(\frac{1}{2}\rho S\right)^2 v_s^4+ {C_{D_b}}^2\left(\frac{1}{2}\rho S\right)^2 v_b^4}$
This can be further simplified to:
$C_D = \sqrt{{C_{D_s}}^2\sin^4\theta+{C_{D_b}}^2\cos^4\theta}$
I don't think this can be simplified further.
Edit: S in this case is of course just the area of the rectangle, I got confused before

Ok, that makes sense to me. Do you have any suggestions on how to add varying surface area? You mention it is more complicated, but I am trying to set up the functions in sympy and am curious where that complexity comes from (and if it's worth looking in to). Looking a what you have written here the surface area for the drag magnitude seems to be where everything blows up in complexity.

Sorry, I got confused, of course S is just the surface area of the rectangle. You could use this, take "long cylinder" for the bottom Cd and "short cylinder" for the side Cd. But tbh the results from this won't be very exact at all. To get a better estimate for the Cd you could for instance use a potential flow method.


text:
Pitot tube model
EDIT: Background on my application
I am making a few measurement models for an extended Kalman filter (EKF). The EKF is meshing an INS and a GPS currently, but the GPS update rate does not update fast enough to keep the filter converged. So I am tasked with adding some other measurement systems so that I can increase it's performance. Due to it being a Kalman filter I need the models to be linearized. So currently I want to linearize the Pitot tube model for that purpose. I am hoping if I find a good solution, I can also use it for other models with the same issues. So far, all of the work will be done purely as a simulation.

I'm trying to get a linearized model for a pitot tube, but I've got an asymptote in the linearized equation that I don't know how to deal with.
I have Bernoulli's equation
$P_o = P + 1/2\rho v^2$
solved for velocity.
$v = \sqrt{\frac{2(P_o-P)}{\rho}}$
I would like a linearized equation so that I can use it as a linear model.
I've taken the derivative with respect to $\Delta P$
$\Delta P = P_o - P$
so that
$\frac{dv}{dt} = \frac{1}{\sqrt{2\rho \Delta P}}\delta \Delta P$
as $\Delta P$ approaches 0 I get an asymptote that I wouldn't get in the nonlinear equation.
My questions are:
1) Is this the proper approach to linearizing this equation?
2) How do I deal with an asymptote, in what should be the physical realm with no disconnect?

Could you elaborate on your application, sound’s interesting

I am making a few measurement models for an extended Kalman filter (EKF). The EKF is meshing an INS and a GPS currently, but the GPS update rate does not update fast enough to keep the filter converged. So I am tasked with adding some other measurement systems so that I can increase it's performance. Due to it being a Kalman filter I need the models to be linearized. So currently I want to linearize the Pitot tube model for that purpose. I am hoping if I find a good solution, I can also use it for other models with the same issues. So far all of the work will be done purely as a simulation.

I am not sure, I understand what exactly are you trying to achieve, but I can try to answer your questions.

Is this the proper approach to linearizing this equation? All depends on purpose, but one usually takes a tangent at the point of interest as a linear approximation around this point. Which is what you did though your notation looks bit strange to me (what is dt for example?).
Anyway, you wonder why this does not work around the point $\Delta P = 0$, right? Take a look at the graph of $\sqrt{x}$, the tangent is "vertical" (parallel to $y$ axis) at the zero. So it does not cover any range of pressures and can not represent (approximate) any other close values of pressure.
The idea of linearization starts with expansion of the function $v(\Delta p)$ in proximity of some point $\Delta p_0$ into series:
$$
v(\Delta p) = v(\Delta p_0 + \varepsilon) = v(\Delta p_0) + \left.{dv\over d\Delta p}\right|_{\Delta p_0}\cdot\varepsilon + \left.{d^2v\over d\Delta p^2}\right|_{\Delta p_0}\cdot\varepsilon^2 + \left.{d^3v\over d\Delta p^3}\right|_{\Delta p_0}\cdot\varepsilon^3 + \cdots
$$
If (!) for your small enough $\varepsilon$ the terms with higher derivation are much smaller than first derivation, you can neglect them and use linear term only.
Note that this assumption does not stay for square root in proximity of zero. In that case the higher derivations have higher value and can not be neglected. You can simply see it looking at the mentioned graph. If you place tangent line to square root graph, the line will stand away.
Depending on particular use, you can be better with using secant instead of tangent. You will get non-zero error in proximity of your chosen reference point, but overall error in larger surroundings can be smaller.
How do I deal with an asymptote, in what should be the physical realm with no disconnect? You simply do not try to approximate square root function with tangent through the origin. Use another suitable line (secant) minimizing overall error if you need linear approximation close to the origin.
This has nothing to do with physics though, the world is not guaranteed to be linear, neither in small scales.

Hello, would it be possible to explain the secant approximation? I've only ever dealt with using a tangent line to approximate a nonlinear equation.

Also for clarification I have test data that changes over time. I consider $\Delta P$ to be time variant with the data. So I take the derivative $\partial v/\partial \Delta P$ which is the first derivative. Then I multiply the error which is an error (in my mind) that is with respect to time. That is how I rationalize it.
essentially the form I have is $dv/dt = \partial v/\partial \Delta P * \partial \Delta P/ \partial t$


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Are there general accuracy requirements for aircraft power supplies?
Let's say I have some LRU device that requires some DC voltage to be powered on. Are there any general requirements in aviation/aerospace that say how accurate that supply voltage must be in order to be allowed on a plane?
Update:
To be more specific, consider an LRU that takes 28V on the input and provides it to a DC-DC converter. I would like to know how accurate that 28V being provided, needs to be.

For the updated question, the LRU (if it's a cots equipment) probably has established specs for its input power. 704E/F etc. so the spec data sheet should contain the info. If not, it's kind of a tricky subject. Are you trying to develop a  power distribution system? It would help receive even more practical answers.

@GürkanÇetin Thanks, that's a good bit of information for when dealing with a specific LRU. Also, no I am not trying to develop a power distribution system. I am just asking a general question here as to whether or not there is a global set of requirements for input power that applies to all LRUs (outside of the spec data sheet for a particular LRU).

Sorry for noob question, but could you define "LRU" ? Are you talking about this ? If so, what type of equipement can that be ? That's unclear to me.

@kebs No problem, I'd be happy to help. As you suggested, an LRU is a line-replaceable unit and that's exactly what I am talking about in this question. Line replaceable units are parts or modules of an aircraft, things like fuel gauges or inertial measurement devices that contribute to operating the aircraft.

Short answer:

Operational voltages on the 28 V bus may range from 22 V to 30 V.
Abnormal voltages on the 28 V bus may occur from 0 V to 60 V.

The detailed requirements depend on the manufacturer, the type of plane and the jurisdiction.
USA: MIL-STD-704
US military standard MIL-STD-704(F) Aircraft Electrical Power Characteristics defines electrical properties of military aircraft equipment. For instance the 28 V DC bus is specified as:

A similar standard should exist for commercial aircraft, unless they just apply MIL-STD-704 as well.
Europe: ISO 1540
In Europe there is ISO 1540:2006 Aerospace -- Characteristics of aircraft electrical systems. This standard differentiates by the actual power source:

Category A: TRU (transformer)
Category B: engine driven DC generator
Category R: regulated DC power from active converter

Category A and B are specified identically for their DC characteristics. Category B is the worst case for AC ripples. Category R is the best case, as it is already regulated. Nevertheless, connected equipment needs to cope with the worst case, that is category B (engine driven generator).
ISO 1540 specifies the 28 V DC bus characteristics (summarized) as:

28 V DC bus - Normal operation: 22 V - 30 V
28 V DC bus - abnormal/onset: 0 V - 60 V

Very nice summary. Discusses the power supply inputs to the LRUs present on the voltage bus exactly like I was asking for. Thank you very much.

Cat-Z can go upto 80V for 100ms

It would depend on whether you are connecting directly to the main DC bus (28V presumably) or being powered by a DC/DC converter.
DC/DC converters are required to meet TSO C71 Airborne Static ("DC to DC") Electrical Power Converter in a certified installation.
Many LRUs I have seen are designed to operate on a DC input voltage from 10 VDC - 32 VDC
If the main power bus is AC, you would need to have an AC to DC converter to provide the DC power.
If you are trying to test the LRU, it should have a specified input voltage range.

That document you posted refers to the output of the DC-DC. I want to know about the input power supply voltage to the LRU. Maybe I should have been more specific.

For example, if the DC-DC on the LRU takes in 28V... I want to knowhow accurate does that 28V have to be?

Your LRU should probably accept anything from 10 VDC to 32 VDC as an input unless are being powered by a converter that will provide you with 28 VDC. That converter would need to meet TSO C71 for its output power (which is your input)

I think there is a lot of useful information here but no DC-DC at the LRU input, it's just an LRU being tested with a bench-top power supply. So that's why I am asking about the voltage that goes into the LRU.

So you are testing an LRU - are you trying to just verify it is functional or are you trying to verify the full range of input voltage for the LRU?

I am not asking about a specific LRU here, but more about the voltage line that any LRU present on the plane must utilize for power and what that voltage line's tolerances are to be acceptable to the LRU.

An LRU could be connected to a variety of power busses and different LRUs are going to operate on a different input range depending on its use. The electrical power system is going to be different from aircraft to aircraft based on the OEM design requirements, starter/generator vs alternator, batteries, APU, etc. There isn't any FAA regulation as to what voltage range or tolerances every plane must meet because every plane can be different.


text:
Is there an ARINC label/data that can be associated with a gyroscope's precession?
I am looking to read the precession of a gyroscope off of an ARINC bus, is there an ARINC label that can be associated with the gyroscope's precession value?
My thoughts are, that it might have something to do with the drift angle, flight path angle, and/or the magnetic heading. Am I in the ball-park? Or is there no way to measure the precession values?

Short answer: No easy way. It may be possible for a crew to detect some drift by comparing the IRS mag heading to the magnetic compass.  This does have problems though as the IRS doesn't really 'measure' mag heading.  It works in true heading and calculates mag heading by applying a magvar correction from a table.
Modern aircraft do not have free gyros that output data. They have inertial reference systems (IRS). The IRS has multiple gyros and accelerometers and outputs a complete navigation solution.  They also have an internal GPS (or an input from an external GPS) to improve their accuracy.  So there's not a lot of 'drift' in the output solution.
These units have internal integrity monitors and will report a fault to the crew if the nav solution exceeds limits.  In some failure modes the IRS will revert to an attitude only mode (ATT) that just outputs raw rates and accelerations (free-gyro mode).  In this case, the crew can provide a heading update to set a heading.  This will drift just like a stand-alone directional gyro.  The crew will have to check it regularly against the mag compass and update as necessary (~ every 10 minutes).


text:
Are the discrete I/O considered to be part of the ARINC interface?
A typical connection used to transmit/receive ARINC data might consist of a cable and a connector similar to the one shown below. Although ARINC transmit/receive channels aren't the only thing this interface can be used for. I've noticed that several different varieties of LRU also use some of the pin connections for I/O discretes.

So, are the I/O discretes considered to be part of the ARINC interface? Or do they just coincidentally share the same connector and have nothing to do with ARINC?
Update
As one of the answers commented on... I am not talking about the discrete bits within an ARINC word but the physical discrete I/O voltage signals that you'd see on an avionics bus.

With ARINC 429 being a single transmit/multiple receiver bus, that connector probably has 4 A429 transmit (8 pins), 8 A429 receive (16 pins) and 6 discretes (6 pins).  I've dealt with multiple 100 pin connectors where 2/3 were A429.

These Mil Std connectors are used on most LRUs for reasons stemming from DO-160. ARINC 429 data lines get passed as one of the many pins on the connector. The other pins may be discrete, analog, or digital signals for different inputs or outputs that aren't even related to ARINC 429.

@selectstriker2 The majority of avionics are ARINC 700 series (what Boeing and Airbus use) and don't use MIL-STD connectors. They use ARINC 600 blind mate connectors - they're on the rear of the units and mate when the LRU is slides into the mounting tray.  They are 'locked' when the LRU is locked into the tray.  Boeing and Airbus do use the MIL-STD connectors in the wiring harnesses at bulkheads,etc.  There are some non-ARINC 600 LRUs typically outside the EE Bay where they do use MIL-STD connectors as they don't want the added weight of the racking system.

@Gerry Once again... Was asking about ARINC in general, but ARINC-429 tag was the closest I could get.

@Snoopy  Agreed.  I was commenting on selectstriker2's comment about the use of MIL-STD connectors.  He said they were used on most LRUs.  I pointed out that the majority of ARINC LRUs use the ARINC 600 connectors.  MIL-STD connectors are used in the harnesses and in some remote LRUs, but I wouldn't say they are used in the majority.

@Gerry I had meant to say "in my experience" but SE wouldn't allow me to modify the comment. It was incorrect to for me to say "most" as that was an assumption. The avionics I have worked with have either used a similar style MIL-STD connector if they would be placed in a harsh environment or a D-Sub if they might be installed in the panel.

@Gerry I agree with you there, I've seen the MIL-STD on some of the flight display modules and fueling stuff but that's about it...

@Gerry, I'm assuming from your description and a quick look at ARINC 600 connectors that those are used in rackmount systems in transport aircraft where extremely high density connectors are needed.

@selectstriker2  They are used in rackmounts and are primarily in the EE Bay, though a few are scattered around the aircraft - mostly RF systems where they want to minimize antenna cable lengths.  Pin density can be fairly high, though some have relatively few connections.  One benefit of the ARINC 600 connectors is that they can handle different cable/connector types.  You can mix twisted pair, shielded twisted pair, coax, quadrax. power and fiber-optic connections within a single shell by using different inserts.  They also simplify and speed line replacement.

It depends on which ARINC document you're talking about. ARINC 429 is a digital serial bus standard. Discrete data may be sent in ARINC 429 data words, but that's just bits in a word not the physical discretes you're asking about.
ARINC also has documents that cover standards for other interfaces as well as 'Characteristics' that define specific LRUs such as ARINC 735A for TCAS II.  In these documents ARINC does specify discrete interfaces.  There are two types, open/ground and 28V/open and are defined for both input and output.
So yes, there are discrete interfaces that are part of ARINC standards, just not part of ARINC 429.

Yeah... So what I wanted to do for this question was ask about ARINC in general, i.e. 429/600/717. However, because my reputation is too low I had to pick the 429 tag because it is the closest thing. Really I would like to be able to use a generic ARINC tag for this.

Also, thanks for pointing out that I'm talking about the physical discretes. That is exactly what I meant. It's good you were able to pick that out but I may still need to update my question to clarify a bit further.

One thing to note, ARINC is a standards and services company owned by Rockwell Collins. So a generic ARINC tag wouldn't really be applicable for what you are asking. I do know that we don't follow any sort of defined standard for our discrete I/O's but they do go through DO-160 testing.

@selectstriker2 Well it is applicable because I'm the one who decided to ask about ARINC in general... why would you assume that I'd know to ask about some specific ARINC protocol?

@selectstriker2 While Rockwell-Collins did buy ARINC, it did so to obtain their data link services company.  Since R-C is an avionics company, owning the standards committees would be a conflict of interest. As a result, R-C sold the ARINC Industry Activities portion of the company to SAE (Society of Automotive Engineers). So ARINC-IA, the producers of ARINC Standards and Characteristics, is not the same as ARINC.

@Gerry Quite true, I didn't realize that the standards portion had been split. Historically though, ARINC was the company publishing the various ARINC standards. My main point was a response to Snoopy's initial comment that I don't feel that a general ARINC tag without the standard number(s) wouldn't be appropriate.

@selectstriker2 I don't disagree with your position on the tag.  There's only 8 questions with the ARINC-429 tag and no other ARINC type tags.  While there are over 100 ARINC standards, I doubt there would be enough questions against them to justify a tag.  The Avionics tag works well for these type questions.  If there was interest, it might be more appropriate to have a general Industry Standards tag.

This is an old question, but as I've stumbled, others will too, and the comments seem to have deviated a bit.
Yes the physical discrete pins are part of ARINC standards, and their electrical specs are defined in subsection 2.2.1 of ARINC 628-1 (newer) and 4.1.4 of ARINC 726 (old) standards.
The question is if they're "part of the ARINC interface" - but ARINC standards define a lot of interfaces and systems, and "arinc interface" will not point to one specifically.

Not all discretes will follow an ARINC standard though, unless they are specifically for equipment conforming to an ARINC standard or using an ARINC defined connector (like the ARINC 600 series connectors @Gerry mentioned)


text:
Does the sterile cockpit rule mean flight attendants could not inform the pilots if a passenger is in the lavatory while on final?
Context:
I've seen this interesting question just the other day, asking what would pilots do after receiving this information while on final:
either go around or continue with the landing.
However many others mentioned the Sterile cockpit rule under 10,000ft. Meaning that they could not discuss this issue.
So what would this mean exactly? The flight attendants could inform the pilots but they can’t respond?? Or the FA’s just can’t/won’t inform the pilots about the situation at all?

I'm now made curious as to what is the risk to this passenger. Is it merely a reduced chance of surviving a crash and an increased chance of being hurt by a very bumpy landing? Or is there a particular situation that might result in him getting covered in sewage?

"Sterile Cockpit" refers to the concept that pilots should not discuss anything not related to the flight during certain phases of flight (often defined as below FL100).
A passenger occupying the lavatory while the flight is on approach and passengers are supposed to be in their seats definitely does have an impact on the flight, so it does not fall under the "sterile cockpit"rule.

Or better said: is one of the topics to be discussed in sterile cockpit...

Just curious, why is a passenger in the lavatory (when they're not supposed to be) relevant to the flight? Is it because of passenger safety?

@tsg Feel free to edit

@a25bedc5-3d09-41b8-82fb-ea6c353d75ae I guess if passengers are supposed to be in their seats, they are also supposed to have their belts fastened. In the lavatory there aren't any belts to fasten...

@a25bedc5-3d09-41b8-82fb-ea6c353d75ae It would affect the center of balance of the plane, and that would alter how it flies. This might be a very significant difference on a very small plane.

@a25bedc5-3d09-41b8-82fb-ea6c353d75ae Your username might be the result of a very old bug: https://meta.stackexchange.com/questions/135178/account-association-has-given-me-a-random-username

@nick012000 a single passenger is not likely to affect weight & balance on an aircraft large enough to have lavatories. There are several questions here at [Aviation.se] about moving passengers to adjust W&B, and most of them agree that it would take large numbers of pax moving to have any impact.

@FreeMan Indeed, and in any case that passenger might be seated right next to the lavatory, or at the opposite end of the plane.

"Sterile cockpit" doesn't mean abject silence; it means no idle chitchat.
An issue relating to the safety of passengers is not idle chitchat, so it can be discussed at any time during the flight.

Just a slightly different wording to hammer it home!

"Sterile cockpit" means also that the cockpit crew shall only perform the duties that are required for the safe operation of the aircraft. Any paperwork, work on the pedestal (the table on AIRBUS) and non-essential conversations between the cockpit members and crew should be avoided.
But:-

During take-off: from the time the wheels are rolling until landing gear is up and the chime sounds, the cabin crew shall not contact the cockpit crew.
During landing: The cabin crew can contact the cockpit crew at any time during the approach, but when the landing gear is extended until aircraft lands and reaches the taxi speed, the cabin crew shall not contact the cockpit crew.

It will all depend on when that passenger decides to hide in the lavatory, but I doubt he can. Cabin crew will not allow him to.

In all the millions of times I've been on a plane, I've never actually noticed; Do the the toilet doors have central locking? That is, when they announce seat-belts on and no further toilet use, can they centrally lock the doors so that no-one can dash to the loo?

@OscarBravo Every plane I've been on (mostly 737s, 747s and the like) the locks have at least appeared to be a local, physical mechanism with no evidence of any machinery that could make central locking work.

@OscarBravo I don't believe there's central locking but the cabin crew can lock them from the outside and there is often a member of cabin crew sat near the w.c. during landing.

@OscarBravo The lavatory doors can easily be locked and unlocked from the outside. The cabin crew does so manually after takeoff and before landing.

You have got the answer before but I would like to add that some airlines have more restrictions on talks between cabin and flightdeck.
For example, in my airline after the call from flight deck to the cabin for cabin crew to be seated for landing, there should be no call from the cabin until landing, even if they spot a fire or something else.


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Are Lycoming O-360-A4M engines generally known to have right magneto issues at low power settings?
TLDR - I am renting a PA-28 181 (engine Lycoming O-360-A4M), and have noticed that the right mag is rough at low power settings. During before takeoff power checks, both magnetos operate as expected, but when making dead cut checks both after start and prior to shut down, the right mag consistently runs rougher than the left - is this normal behaviour?
CONTEXT - I've been told, by the club that I am renting this aircraft from, that the engine has always been like this, and that it is characteristic behaviour, but I've been becoming a bit more dubious as the problem seems to be worsening, and I will now often get backfires and VERY rough running (600-900 rpm) on the right mag during dead cut checks. The left mag runs perfectly smoothly during dead cut checks at about 1100 rpm (I conduct dead cut checks with the engine at 1200 rpm for reference). The engine also seems very reluctant to settle at 1200 rpm before power checks are conducted, and consistently jumps between running fast at 1500 rpm and then running unevenly at about 900 rpm. After power checks are made, the problem seems to rectify itself, and the engine will happily run at 1200 rpm.
FIXES - I can't find anything in the POH with this set of symptoms, and dead cut checks do not specify maximum rpm drops / tolerances (although for reference, I can tell you that it is uncomfortable sitting in the plane when the right mag is isolated during dead cut checks). Carb heat issues can be largely ruled out, since I always check this on the ground. Fouling seems unlikely given that the plane is always leaned for taxi, usually leaned at cruise, never flown especially high, and never flown or taxied at low power settings for extended periods (it flies out of a small airport, and off block to take off time is never more than 5 minutes).
I've mostly flown the PA-28 161 rather than 181, and have not flown enough different airfames of this type to know if this kind of anomaly is harmful or not, although another pilot not from this club has expressed concerns. I would much appreciate any advice!

I'm not a certified A&P, but I do know that Lycoming engines have an impulse coupling on the left magneto only. This is why you perform startups on the left. Magnetos naturally have performance issues at low RPM. Perhaps the quick power cut check is short enough for the impulse coupling to keep the left firing smoothly, while the right mag struggles under the low RPM.

When is the last time the mag was pulled and checked? We just had a mag on our aircraft experience similar issues, and the mag was really bad. Our ignition switch was also shot.

Unfortunately I don't manage the maintenance, so I don't know when the mags were last checked, but something wrong with the right mag was my first thought. The aircraft just had its annual, but not being the engineer, I don't know exactly which parts are checked during that. I do know that the engine is old but low hours however. Could you please clarify what you mean by "similar issues"?

We had a rough mag on one side, originally we thought it was just fouled plugs on the ground or using carb heat at low RPM (in the pattern). Eventually it started misfiring quite a bit and was followed by a flight that had a loss of power and inability to climb (luckily they made it back to the field). The aircraft had an annual 2 months prior, they pulled the mag and found it had really bad contacts. As far as I know the dead-mag checks worked fine (RPM drop was OK), but it would run rougher on one mag than the other.

Just to be clear, the mag was not pulled at annual, the rough engine existed before the annual and got worse afterward. They didn't check the mag at annual and we were a little upset about that, but I don't think it's a required annual item. Some mags are life limited parts

The 0-360 like most all aircraft engines have a set of rpm drops that should be close to each other ie 50-75 rpm. The symptoms mentioned are signs of a bad mag or possible plugs. During an annual inspection the mag timing is to be checked but not to include a tear down inspection. All mags should have a 500 hr inspection done that requires them to be pulled and inspected. Sadly it is optional for most owners and so not done.

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